Da Orky Man said:
I think that Niccolo Machiavelli was right, at least in a broad sense.
Continuing from that, I think that people who don't believe that "The ends justify the means" are incredibly selfish. Say that you had to kill a child soldier to prevent a bomb being set off in the middle of London/Washington/Moscow, or anywhere with a large concentration of people. Would you honestly allow hundreds of people to die just to allow your conscience to be clean?
As one of those people, I think you may misunderstand the position. If I say "The ends do not justify the means" or "You cannot commit evil with good intentions", that doesn't mean you can't do unpleasant things, but that you cannot do things which are, in and of themselves, always evil. Like killing an innocent. So, if your situation is that there is a child soldier trained to kill, in the middle of an operation which will result in a bomb going off killing thousands, then I would say that that soldier, child though he may be, is not an innocent. He's an enemy combatant. By all means, stop him by any means necessary, up to and including lethal force.
Then again, you may be talking about the classic "ticking time bomb" moral question. If that's the case, then my answer is yes, I would "allow" those people to die, rather than committing some monstrously evil act. How that is considered selfish, when in all likelihood I or my loved ones may be within the blast area, is a mystery to me.